As a detection apparatus capable of estimating density of target objects, there is for example the quantitative fish finder disclosed in JP3930166B that estimates density of objects (fish). Specifically, the quantitative fish finder has a transmission module that transmits a transmission signal having a long pulse width and a transmission signal having a short pulse width, and calculates a density distribution of a fish school (target object) based on a long pulse reception signal and a short pulse reception signal obtained from reflection waves from each transmission signal transmitted by the transmission module. In this quantitative fish finder, the fish school density is calculated by dividing a volume backscattering strength per unit volume of the fish school calculated based on the long pulse reception signal by a target strength of single fish calculated based on the short pulse reception signal.